Ecological risks posed by emerald ash borer to riparian forest habitats: A review and problem formulation with management implications

The exotic, invasive forest insect pest, emerald ash borer (EAB), is rapidly spreading throughout eastern North America and killing almost all ash trees in its path. The loss of ash from forest habitats could trigger a cascade of ecological effects on habitat quality and the biological communities associated with it. Riparian forest habitats serve critical and often unique functions because they can exert a disproportionate influence on the productivity of riparian soils and adjacent aquatic ecosystems, and can provide important residual habitats in human-influenced landscapes. When ash trees are present in riparian forests, the rapid loss of ash from EAB infestations could put those unique riparian forest functions at risk. The first step in assessing risks and predicting outcomes of threats to an ecosystem is to formulate predictions from existing knowledge. We briefly review the literature on riparian forest ecosystem function and the impacts of other insect pest species on riparian structure and functional processes, with an emphasis on the risks to nutrient subsidies from riparian forests to adjacent waters. We then present a problem formulation that predicts impacts of the loss of ash on riparian forests. We provide a theoretical bases for predicting that most adverse ecosystem effects will arise from reductions in high-quality leaf litter inputs as nutrient subsidies to consumer communities and from the large canopy openings as a result of the rapid loss of ash in riparian forests. Management guidelines to address these potential effects are suggested, but we recognize that actual, empirical studies to measure and assess the ecological impacts of EAB-induced loss of ash from riparian forests would greatly improve risk predictions and management responses.